Systems, methods, and/or apparatuses for non-invasive monitoring of respiratory parameters in sleep disordered breathing

ABSTRACT

In certain example embodiments, an air delivery system includes a controllable flow generator operable to generate a supply of pressurized breathable gas to be provided to a patient for treatment and a pulse oximeter. In certain example embodiments, the pulse oximeter is configured to determine, for example, a measure of patient effort during a treatment period and provide a patient effort signal for input to control operation of the flow generator. Oximeter plethysmogram data may be used, for example, to determine estimated breath phase; sleep structure information; autonomic improvement in response to therapy; information relating to relative breathing effort, breathing frequency, and/or breathing phase; vasoconstrictive response, etc. Such data may be useful in diagnostic systems.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/514,696, filed on May 13, 2009, now allowed, which claims the benefitof U.S. Provisional Application No. 60/858,414, filed on Nov. 13, 2006,the entire contents of which is hereby incorporated herein in itsentirety. This application incorporates by reference the entire contentsof each of PCT Application No. WO 2006/037,184, filed on Oct. 6, 2005,and U.S. Provisional Application No. 60/615,961, filed on Oct. 6, 2004.

FIELD OF THE INVENTION

The invention relates to monitoring parameters relevant to SleepDisordered Breathing (SDB).

BACKGROUND OF THE INVENTION

Sleep Disordered Breathing (SDB) has been traditionally identified asbeing associated with Obstructive Sleep Apnea (OSA) and Cheyne-StokesRespiration (CSR). Today there are a number of other conditions alsorecognized as being associated with SDB including, e.g., cardiovasculardisease, stroke and diabetes, etc. Patients with these conditions andSDB may benefit from the treatment of their SDB with positive pressureventilatory support by some form of mechanical ventilator.

While basic nasal Continuous Positive Airway Pressure (CPAP) ventilatorsmay not monitor their patients, in general, the patients benefit fromhaving a device which monitors the patients as part of some kind ofcontrol loop. In particular devices are known to monitor pressure, flowand patient effort.

An existing problem for known devices includes discriminating betweenobstructive sleep apnea (OSA) and central sleep apnea (CSA). OSA isindicative of upper airway collapse and can be used as an input toauto-titration algorithms for the CPAP pressure applied or theend-expiratory pressure (EEP) used in a bi-level device. CSA can beindicative of over-ventilation and can therefore be used as an input toalgorithms that auto-titrate the ventilation of the patient. Clearly,miscategorizing an apnea as either closed or open results in thesetitration algorithms prescribing sub-optimal parameters for thetreatment of the patient.

Obstructive and central sleep apnea are discriminated in known devicesby injecting a 1 cm peak-to-peak 4 Hz oscillation into the treatmentpressure waveshape and measuring the resulting 4 Hz flow. The generalterm for this technique is Forced Oscillation Technique (FOT). Thephasic difference in the flow to the pressure waveshape is indicative ofthe compliance of the load which is then used to deduce if the upperairway is opened or closed. Unfortunately, this method does not give anyinformation on events that include upper airway narrowing/closure andsimultaneous central sleep apnea.

Obstructive and central sleep apnea are also discriminated in knowndevices by detecting the cardiogenic flow. The cardiogenic flow is theairflow induced in the lungs during a heart beat due to the proximity ofthe lungs to the heart. During OSA, there is therefore never anycardiogenic flow. Like the previous solution, it is also unable todetermine if CSA and OSA have occurred concurrently.

Another existing problem for known devices includes inferring highpatient respiratory effort. Patient respiratory effort is a keyindicator used by clinicians when evaluating the acute state of apatient in a number of diseases including sleep apnea, obstructive lungdisease, and various restrictive diseases. Despite its known value, ithas not enjoyed widespread use as either an input to flow generatortitration algorithms or as a recorded clinical parameter due to theinconvenience or impracticality of the transducers involved.

The “gold standard” in terms of accuracy for monitoring effort is anoesophageal catheter which a patient is required to swallow.Unfortunately, this is uncomfortable and awkward for a patient and notpractical outside a clinic. Respiratory bands around the patient's chestand abdomen are known to monitor effort. Suprasternal notch effortsensors are also known, as well as the use of EMG and ECG sensors. Thesetechniques are all unsuitable for home use.

Another existing problem for known devices includes measuring andstoring vaso-specific parameters, such as cardiac afterload, vasculartone, heart rate variability, sympathetic nervous system activity ingeneral, and/or central venous pressure. If these parameters wereavailable in real-time in a flow generator, they could be used to (a)contribute to auto-titration algorithms and (b) be recorded withrespiratory specific parameters to allow physicians to observe long-termtrends and have a richer data set to determine the long term managementof the patient.

Yet another existing problem for known devices includes limiting themean mask pressure. Auto-titrating CPAP algorithms aimed at eliminatingOSA or upper airway resistance syndrome (UARS) may use breath flowanalysis to limit upper airway narrowing. Pressure beyond certain levelsmay, in some patients, be deleterious to cardiac function. Equally, alower pressure may be beneficial to cardiac function provided it doesnot result in complete closure of the upper airway (e.g., a lowerpressure may promote UA closure). It is desirable to includecardiovascular parameters in auto-titration schemes such thatrespiratory therapy (e.g., CPAP pressure) can be continuously optimized.Such parameters may include cardiac afterload, vascular tone, heart ratevariability, sympathetic nervous system activity in general, and/orcentral venous pressure if they could be acquired non-invasively andconveniently.

ResMed's AutoSet CS and AutoSet CS2 devices specifically target patientswith heart disease. These devices address the ‘excessive CPAP pressure’problem by imposing a maximum average pressure of 15 cmH₂O.

Another known sensor is a suprasternal notch effort sensor. See U.S.Pat. No. 6,445,942 (Berthon-Jones). Other known techniques formonitoring apneas and hypopneas are described in U.S. Pat. No. 6,091,973(Colla et al.) and U.S. Pat. No. 6,363,270 (Colla et al.). Anotherrelated U.S. patent is U.S. Pat. No. 5,704,345 (Berthon-Jones) whichdescribes distinguishing open and closed airway apneas amongst otherthings. U.S. Pat. No. 6,484,719 (Berthon-Jones) describes aservo-ventilator which uses a flow sensor. The contents of all thesepatents are hereby expressly incorporated by reference herein.

Thus, a need has developed in the art to overcome one or more of theseand other disadvantages.

SUMMARY OF THE INVENTION

One aspect of the present invention relates to therapy throughventilator optimization through one or more of improving synchrony,pressure support/volume autotitration, reducing side-effects (e.g.,excessive EEP); and patient management (such as, for example, trendingrespiration, cardiac/autonomic function, sleep structure, endothelialfunction, etc.).

Another aspect of the present invention relates to therapy through CPAPoptimization, including one or more of OSA/CSA discrimination, reducingside-effects (e.g., excessive EEP), auto-titration, and patientmanagement (such as, for example, trending cardiac/autonomic function,sleep structure, endothelial function, etc).

Still another aspect of the present invention relates to monitoringand/or diagnosis. SDB diagnosis and patient management may be achievedvia oximetry alone or in combination with respiratory flow (apnealink).SDB diagnosis may include both OSA and CSA.

Yet another aspect of the present invention relates to monitoring and/ordiagnosis via early detection of exacerbations in respiratory disease(e.g., trending).

An aspect of the present invention relates to monitoring and/ordiagnosis by monitoring of autonomic function, sleep quality,respiratory timing/effort, and vascular tone (e.g., arterial stiffness)in general (including, for example, patient sub-groups such as cardiacfailure, stroke, hypertension, pediatrics, obesity-hypoventilationsyndrome, motor-neurone disease, etc.).

It will be appreciated that in any of the above, the PPG information maystand alone, or be combined/correlated with respiratory flow ortraditional SpO₂ from the oximeter or with other respiratory monitorssuch as trans-cutaneous CO₂. It also will be appreciated that thetechniques for monitoring, detection, and treatment may be used alone orin any combination.

Certain example embodiments provide for an air delivery system,comprising a controllable flow generator operable to generate a supplyof pressurized breathable gas to be provided to a patient for treatment;a pulse oximeter configured to generate, during a treatment period, apatient effort signal for input to control operation of the flowgenerator; and a controller configured to derive an estimated breathphase of the patient independent of measured flow, based at least inpart on the patient effort signal.

In certain example embodiments, a method for treating sleep disorderedbreathing is provided, with the method comprising deriving a pulseoximeter signal from a patient; processing the pulse oximeter signal togenerate a patient effort signal indicative of respiratory rate; andderiving an estimated breath phase of the patient independent ofmeasured flow based at least in part on the patient effort signal.

In still other example embodiments, a respiratory effort monitoringapparatus is provided, comprising a pulse oximeter configured to derivea pulse oximeter signal; and a signal processor configured to receivethe pulse oximeter signal and generate signals indicative of respiratoryeffort. The apparatus can be used in conjunction with a method fordiagnosing SDB, respiratory disease (including asthma), and/or cardiacfailure (e.g., periodic breathing). The signal processor may beconfigured to derive an estimated breath phase of the patientindependent of measured flow based at least in part from the patienteffort signal.

In other example embodiments, an air delivery system comprises acontrollable flow generator operable to generate a supply of pressurizedbreathable gas to be provided to a patient for treatment; a pulseoximeter; and a controller configured to determine sleep structureinformation for the patient is tracked to indicate a therapy'seffectiveness based at least in part on output from the pulse oximeter.

Still other certain example embodiments provide a method for diagnosingsleep disordered breathing, with that method comprising deriving a pulseoximeter signal from a patient; processing the pulse oximeter signal togenerate a patient effort signal indicative of respiratory rate; andtracking sleep structure information for the patient to indicate atherapy's effectiveness.

Certain example embodiments provide a respiratory effort monitoringapparatus, comprising a pulse oximeter configured to derive a pulseoximeter signal; and a signal processor configured to receive the pulseoximeter signal and generate a patient effort signal indicative ofrespiratory rate; wherein the signal processor is configured to tracksleep structure information for the patient to indicate a therapy'seffectiveness.

Certain other example embodiments provide an air delivery system,comprising a controllable flow generator operable to generate a supplyof pressurized breathable gas to be provided to a patient for treatment;a pulse oximeter configured to determine a measure of patient effortduring a treatment period and provide a patient effort signal for inputto control operation of the flow generator; and a controller configuredto extract information regarding patient-ventilator asynchrony from thepatient effort signal, and to measure and/or monitor the patient'sautonomic improvement in response to therapy.

Still other example embodiments provide a method for monitoringcardio-respiratory data associated with sleep disordered breathing, withthe method comprising deriving a pulse oximeter signal from a patient;processing the pulse oximeter signal to generate a patient effort signalindicative of respiratory rate; measuring and/or monitoring thepatient's autonomic improvement in response to therapy; and extractingpatient-ventilator asynchrony from the patient effort signal.

In certain example embodiments, a respiratory effort monitoringapparatus is provided, comprising a controllable flow generator operableto generate a supply of pressurized breathable gas to be provided to apatient for treatment; a pulse oximeter configured to determine ameasure of patient effort during a treatment period and provide apatient effort signal for input to control operation of the flowgenerator; and a controller to extract patient-ventilator asynchronyfrom the patient effort signal, and to measure and/or monitor thepatient's autonomic improvement in response to therapy.

In certain other example embodiments, an air delivery system forclinical management and/or prediction of respiratory exacerbations isprovided, comprising a controllable flow generator operable to generatea supply of pressurized breathable gas to be provided to a patient fortreatment; and a pulse oximeter; wherein the controllable flow generatorand/or the pulse oximeter are configured to determine informationrelating to relative breathing effort, breathing frequency, and/orbreathing phase.

In still other example embodiments, a method for clinical managementand/or prediction of respiratory exacerbations is provided, the methodcomprising deriving a pulse oximeter signal from a patient; andprocessing the pulse oximeter signal to determine information relatingto relative breathing effort, breathing frequency, and/or breathingphase, with or without standard oximeter metrics such as oxygensaturation and average heart-rate.

Yet other example embodiments provide a respiratory effort monitoringapparatus for clinical management and/or prediction of respiratoryexacerbations comprising a controllable flow generator operable togenerate a supply of pressurized breathable gas to be provided to apatient for treatment; and a pulse oximeter; wherein the controllableflow generator and/or the pulse oximeter are configured to determineinformation relating to relative breathing effort, breathing frequency,and/or breathing phase.

Certain example embodiments provide an air delivery system withprovision for assessment of endothelial dysfunction comprising acontrollable flow generator operable to generate a supply of pressurizedbreathable gas to be provided to a patient for treatment; a pulseoximeter configured to measure the patient's vasoconstrictive responseto treatment; and a controller configured to trend the vasoconstrictiveresponse over a given time period to indicate a change in endothelialfunction.

Still other example embodiments provide a method for assessment ofendothelial dysfunction, the method comprising deriving a pulse oximetersignal from a patient; processing the pulse oximeter to measure thepatient's vasoconstrictive response to treatment; and trending thevasoconstrictive response over a given time period to indicate a changein endothelial function.

Certain other example embodiments provide a method of monitoringsleep-disordered breathing, the method comprising deriving a pulseoximeter signal from a patient; processing the pulse oximeter signal togenerate a patient effort signal indicative of respiratory rate;measuring saturation variation; and, correlating the patient effortsignal with the saturation variation to detect and/or quantify the levelof periodic breathing by the patient.

Parameters of interest (e.g., cardiac afterload, vascular tone, heartrate variability, and/or central venous pressure, etc.) can be estimatedfrom a pulse oximeter plethysmograph. Currently, pulse oximeters areprimarily employed for monitoring SpO₂ and heart-rate. Some pulseoximeters display a plethysmograph, but as far as is known, none of theinformation present in the plethysmograph is used as input toauto-titrate respiratory or cardiovascular therapies. PeripheralArterial Tone (PAT) is a novel multi-cell finger plethysmography systemthat focuses specifically on arterial tone. This technology may be analternative to pulse oximetry as the sensing modality. Pulse-transittime (PTT) also contains information on autonomic activity and arterialtone.

Each aspect can be manifested in the form of a method and/or apparatusfor non-invasive monitoring of one or more parameters relating to thediagnosis of a patient's health disorder, e.g., sleep disorderedbreathing, congestive heart failure, stroke, respiratory disease, etc.,and/or controlling a ventilator or other respiratory therapy device inaccordance with the monitored parameter and/or the derived diagnosis.

Another aspect of the invention is to monitor a patient using pulseoximeter plethysmography without treating them.

Further aspects of the invention are set out in the attached claims.

Other aspects, features, and advantages of this invention will becomeapparent from the following detailed description when taken inconjunction with the accompanying drawings, which are a part of thisdisclosure and which illustrate, by way of example, principles of thisinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings facilitate an understanding of the variousembodiments of this invention. In such drawings:

FIG. 1 shows a pulse oximeter waveform transformed into an effortsignal.

FIG. 2 shows a range of pulse oximetry applications in accordance withvarious embodiments of the invention.

FIG. 3 shows a therapy system in accordance with an embodiment of theinvention.

FIG. 3A is a schematic diagram of a monitoring system according to anembodiment of the present invention.

FIG. 4 shows an algorithm for Upper-airway obstruction (inspiratory flowlimitation) and Auto-EEP/AutoCPAP in accordance with an embodiment ofthe invention.

FIG. 5 shows an algorithm for Auto-EEP titration/Automated PressureSupport titration in accordance with an embodiment of the invention.

FIG. 6 shows an algorithm for Detection of elevated Sympathetic NervousSystem (SNS) or reduced cardiac output—cardiac patients onCPAP/AutoCPAP/Comfort (fixed low-support bilevel) devices in accordancewith an embodiment of the invention.

FIG. 7 shows an algorithm for AutoCPAP on cardiac patients in accordancewith an embodiment of the invention.

FIG. 8 is a block diagram illustrating a procedure for initializing NPPVtherapy rate settings, based on respiratory rate information, inaccordance with an embodiment of the present invention.

FIG. 9 is a block diagram illustrating a procedure for initializing NPPVtherapy trigger threshold settings, based on positively identifyingcardiogenic flow amplitude, in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Pulse oximeter plethysmography (sometimes referred to simply as “pulseoximetry” or “photo-plethysmogram”) is a standard method of obtainingblood oxygenation data in a non-invasive and continuous manner.Oximeters use two wavelengths of light to solve for hemoglobinsaturation. The waveforms are created by the absorption produced bypulsatile arterial blood volume, which represents the alternatingcurrent (AC) signal. The absorption produced by nonpulsatile blood,venous and capillary blood, and tissue absorption is depicted by thedirect current (DC) signal. See Hartert et al., Use of Pulse Oximetry toRecognize Severity of Airflow Obstruction in Obstructive Airway Disease,Correlation with Pulsus Paradoxus, Chest 1999:115:475-481. A pulseoximeter signal from Hartert et al. is shown in FIG. 1.

Currently, pulse oximeters are primarily employed for monitoring SpO₂and heart-rate; however, in accordance with an embodiment of theinvention, the pulse oximeter is used as an indication of patient effortin a respiratory therapy device. Respiratory effort can be seen in thearterial blood pressure waveform as variation in the peak-to-peakamplitude. This is caused by the effect of the respiratory pleuralpressure swings on cardiac output throughout the breathing cycle.Inspiration is associated with reduced systolic blood pressure, and thisrespiratory effect on blood pressure is referred to as ‘pulsusparadoxus.’

This effect has been proposed as a measure of respiratory loading invarious areas (e.g., asthma exacerbation, obstructive lung disease,etc.), where a variation of >10 mmHg is associated with high respiratoryeffort. The reference standard for measuring arterial blood pressure isinvasive (catheter), so indirect methods are desired. One such method ispulse-transit time (PTT), where the variation in blood pressure causes avariation in vascular compliance, transduced as the propagation time ofthe pulse from the heart to the periphery. Another method is theoximeter plethysmographic waveform, which relates the volume of arterialblood in the tissue bed being sensed (usually finger or ear). Changes incardiac output throughout the respiratory cycle may be seen as variationin the plethysmogram's peak-to-peak amplitude, consistent with thearterial blood pressure observations. This variation in cardiac output,combined with the variation in central venous pressure due torespiration, also induces changes in the baseline/offset of the PPGsignal synchronous with breathing. A third factor seen in the PPG isaffected by breathing: the heart period is also modulated somewhat byrespiration, primarily via the respiratory neural outflow, and to alesser extent in response to the arterial pressure variations induced byrespiration.

Since the pulse oximeter plethysmogram is more related to volume ofblood in the tissues, variation in the baseline/offset of the pulsatilecomponent may be a more sensitive indicator of cardiopulmonaryinteraction than the cardiac output variation (Hartert et al., Use ofPulse Oximetry to Recognize Severity of Airflow Obstruction inObstructive Airway Disease—Correlation with Pulsus Paradoxis; Chest1999; 115: 475-481).

Other factors (e.g., arterial tone, cardiac performance, posturalchanges, etc.) can also cause variations in the PPG, so processing isrequired to analyze the variation over the respiratory frequencies, andcan be aided further by correlating the variation with respiratory flowinformation provided by the flow generator. A progressive increase inPPpleth (pulsus paradoxus from the plethysmogram) over a number ofbreaths (e.g., 3-5) may indicate increasing efforts associated withimpending upper airway (UA) collapse. A dramatic increase in PPplethmight indicate UA obstruction.

The waveform may be characterised into the following categories:

(a) Pulsatile amplitude: The AC amplitude of the pulse is mostindicative of vascular compliance, which is greatly affected by arterialtone/sympathetic nervous system activity when looked at over 20-30seconds or greater (for an example of methods, refer Am J Physiol HeartCirc Physiol 283; H434-H439, 2002). As such, it can indicate arousalfrom apnea, and over many days/weeks, may demonstrate the long-termbenefits of abolishing OSA/UARS on SNS activity. The finger is the bestsite for detecting the effect of autonomic activity on vascularcompliance. Pulse oximetry at the ear is less sensitive to autonomicactivity, but can offer a relative estimate of central blood pressure,given that vascular compliance exerts a lesser effect.

(b) Offset or baseline: Respiration induces a phasic variation in thepulse baseline (pulsus paradoxus) that varies in accordance withrespiratory effort (the pressor response). This effect has been used toidentify airway resistance (asthma) and obstruction (obstructive lungdisease). See Comparison of traditional and plethysmographic methods formeasuring pulsus paradoxus (Clark J et al., Arch Pediatr Adolesc Med2004. 158: 48-51) and use of pulse oximetry to recognize severity ofairflow obstruction in obstructive airway disease; correlation withpulsus paradoxus (Hartert et al., Chest 1999. 115: 475-481. Availableonline at http://www.chestjournal.org/cgi/reprint/115/2/475).

(c) Pulse rhythm/timing: Pulse timing and heart period can shed light onnumerous physiological factors, dealt with in turn below.

Sympatho-Vagal Balance:

Heart-rate variability indices (HRV, traditionally derived from ECGsignals) can be calculated from the pulse period, inferringsympatho-vagal balance as performed routinely in ECG analysis (for anoverview, see Circulation 1996; 93: 1043-1065).

Sleep Structure:

Statistical or fractal analysis of pulse interval data throughout anight can distinguish sleep-wake state. REM sleep is similar to wakeperiods in the fractal component of HRV/heart period, but the non-REMsleep stages differ significantly from awake (seehttp://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=12059594;http://ajpheart.physiology.org/cgi/reprint/280/1/H17;http://ajpheart.physiology.org/cgi/reprint/283/1/H434). HRV data mightbe analyzed to indicate sleep onset, since the patient must pass throughnon-REM sleep prior to achieving REM sleep. Heart-rate can be obtainedvia various monitors (e.g., ECG, standard oximeter, etc.). The PPGinherently contains heart period, and provided this period informationis not averaged, it can be used to conduct traditional HRV analyses.Thus, the PPG signal provides the opportunity to capture unfilteredheart-rate data, in contrast to typical pulse oximeter heart-rateoutputs. One method of discriminating sleep/wake from HRV is taught byIvanov et al., Europhysics Letters 1999, 48(5): 594-6000.

Vascular Tone and Sympathetic Activation:

Simultaneous access to an ECG signal in addition to the oximeter pulsetiming can offer an indication of vascular tone, which can augment thesensitivity or specificity of any conclusions regarding arousal,respiratory effort, or sympathetic tone. The ECG can indicate the momentof electrical systole, so the time delay between this central cardiactiming and the arrival of the pulse at the periphery can define theleft-ventricle's pre-ejection period plus the transit time of the pulsethrough the systemic arteries to the peripheral probe site. This overallduration is the traditional definition of pulse-transit time, or PTT.Pulse transit time is affected by vascular tone (also referred to asarterial stiffness), which can be affected by factors such assympathetic activation and blood pressure. Consequently PTT can be anindicator of arousal (transient increases in sympathetic outflow and BP)and an indicator of average BP/average sympathetic activation whenviewed over longer periods. An alternate means of calculatingpulse-transit time is to replace the timing of electrical systole withmechanical systole, e.g., cardiogenic flow (CGF) seen in a respiratoryflow signal (e.g., acquired during ventilator or CPAP therapy or using anasal pressure transducer), after deducting a fixed propagation delayfrom lung to nares. By combining the timing of the cardiogenicrespiratory flow signal with the timing of the plethysmographic pulse itmay be possible to calculate relative variations in pulse-transit timemore accurately than traditional PTT (pulse transmission time)estimates, since the left-ventricle's pre-ejection period is notincluded in the measured duration (the pre-ejection period is known tosometimes detract from the sensitivity of the ECG-derived PTTmeasurement). In addition, by acquiring the CGF pulse at a consistentplace within the respiratory cycle (end expiration, when it is mostreadily seen), the respiratory-induced fluctuations in PTT can beignored. That leaves just the PTT variations due to either BP variationor changes in arterial tone (sympathetic activation), both of whichshorten the PTT, and both of which are associated with arousal, therebyoffering another important SDB parameter. For an overview of PTT methodsand applications, refer to Thorax 1999; 54: 452-458.

(d) Waveshape: The wave morphology contains similar information to thatseen in arterial catheter pressure signals or applanation tonometry.Some examples include:

Vascular Tone:

For example, the position and relative amplitude of the “dicrotichnotch” (e.g., the bump on the trailing shoulder of the pleth waveform)can point to the degree and timing of pressure-wave reflections of theforward-going wave from the peripheral circulation, and may be itself anindicator of vasomotor tone (SNS). This feature can be trended usingestablished methods: for example, the first-derivative of theplethysmogram is closely related to arterial flow to the area, while thesecond-derivative of the waveform has been proposed as an indicator ofvasoactivity and arterial compliance (e.g., Hypertension 1998; 32:365-370).

Cardiac Congestion/Impeded Venous Return:

Venous pulsation may be apparent in the waveform, which representsinteraction between a number of factors, but in our case may indicatethe effect of excessive CPAP (increased central venous pressure) orimprovement in congestive heart failure (reduced central venouspressure). Certain examples of venous pulsation within the PPG waveformare illustrated in chapter 23 of Clinical Monitoring: Practicalapplications for anesthesia and critical care, WB Saunders & Co, 2001.

Methods for extracting the above parameters from the raw PPG exist,comprising, for example, time-domain or frequency-domain signalprocessing techniques, or elements of both. One example are the methodstaught in WO 03/00125 A1 and WO 2004/075746 A2, employing the continuouswavelet transform to extract the respiratory signals and arterial toneinformation from the raw PPG. Time-domain analysis of assessing baselinefluctuations from the PPG are summarized by Shamir et al, BritishJournal of Anaesthesia 1999, 82(2): 178-81

Recent developments in oximeter signal processing has allowed deviceperformance to be more robust when presented with movement and lowperfusion. Modern embedded processors allow more sophisticatedpost-processing of plethysmographic waveforms, and even the mostadvanced oximeter technology is available as OEM module format. Thesetechnological advances, together with the underlying information presentin the plethysmogram combined with information from the therapy device,may permit a respiratory device to employ an oximeter as part of aservo-controlled therapy.

The information present in the plethysmogram may be useful todiagnosis-only studies as well, since it can indicate arousals that maynot be evident as a desaturation.

Respiratory effects can also be seen as variation in cardiac timing,termed ‘respiratory sinus arrhythmia,’ which may also be used to extractrespiratory timing information.

An aspect of the invention relates to the combination of (1) oximeterplethysmograph-derived parameters with (2) respiratory flow information,to augment real-time control algorithms for a respiratory therapydevice.

This arrangement may prove superior to current techniques if, forexample, it permits a more thorough and timely estimate of the patient'sacute condition allowing algorithms to prescribe more appropriatetherapy. The parameters are also optionally stored within the flowgenerator to give a physician a richer data set for long term patientmanagement. This is superior to current technologies as it gives aphysician data from flow generators used in an unsupervised environmentsimilar to that gained in a sleep study.

Plethysmographic parameters useful for titration and long term patientmanagement include all those noted above (e.g., patient effort, vascularcompliance, heart rate variability, arrhythmia detection, venouspulsation, and SpO2, etc.).

In accordance with an embodiment of the invention, a pulse oximetersignal 10 is fed through signal processor 20, for example, a low passfilter, peak detection, nadir detection or averaging. The filter isdesigned to remove signals indicative of heart rate and leave signalsindicative of respiratory rate.

Once the raw PPG signal is acquired from the pulse oximeter, it may beanalyzed in a number of ways as shown in FIG. 2 and described in furtherdetail below:

(i) Open-Closed Apnea Discrimination. The plethysmographically derivedrespiratory effort estimate can be used during episodes of apnea (usingrespiratory flow data) to indicate whether the apnea is opened(non-obstructed) or closed (obstructed), useful in automatic titrationalgorithm logic. For example, a low or zero flow signal is indicative ofan apnea. If the apnea occurs in the absence of effort as measured bythe pulse oximeter (e.g., no change or reduction in the relative effortsignal), then the apnea is regarded as being “open.” However, if thereis effort (e.g., increase in the relative effort signal), then the apneais regarded as being “closed.”

(ii) High airway resistance. Similarly, a period of high respiratoryeffort derived from the oximeter plethysmograph (e.g., increase in therelative effort signal) combined with unchanged or reduced respiratoryflow, or combined with flow limitation (inferred by flow waveshape, astaught, for example, in U.S. Pat. Nos. 5,704,345, 6,920,877, and6,988,994, each of which is incorporated herein by reference in itsentirety) can imply the presence of significant airway resistance, be itdue to expiratory flow limitation or upper-airway resistance. In bothcases, the combination of high relative effort with unchanged or lowmeasured respiratory flow may be an indicator to increase applied PEEP.

(iii) Relative work of breathing: In the absence of respiratory flowlimitation (adjudged from respiratory flow waveshape or estimatedvolumes), persistently high respiratory effort may indicate inadequatepressure support (under-ventilation).

(iv) Used in conjunction with a flow based measure of phase (such asdescribed in U.S. Pat. No. 6,484,719, which is incorporated herein byreference in its entirety).

(v) Using the relative effort information to augment ResMed's AutoSetCPAP algorithm. Increasing patient effort (e.g., increase in relativeeffort over 3-5 breaths) is indicative of impending upper-airwayinstability. AutoCPAP titration based on increased patient effort may bemore pre-emptive of obstruction than the current flattening basedalgorithm.

(vi) Using the effort information as a basis for an algorithm in aResMed's VPAP or AutoCS device to titrate applied PEEP. It isconceivable that titration algorithms based on inspiratory waveshapewill be challenged when used in devices that change the pressure duringthe breath cycle. Changes in patient effort may not be as dependent onintra-breath changes in pressure and hence may be more robust to thesetypes of therapy.

(vii) Using the relative effort signal as an early indicator that apatient has been overventilated. This may be a possible consequence ofinappropriate servo-ventilation, where a ventilator augments thepatient's ventilation to achieve a target level. This indicator can beused to titrate the target ventilation.

In addition, by offering an alternate estimate of breath phaseindependent of measured flow, spontaneous breath phase may be moreaccurately assessed, permitting, for example:

-   -   Indication of patient-ventilator asynchrony, useful for acute        ventilatory configuration, for assisting clinical management of        chronically ventilated patients, etc.    -   Predictive breath-phase algorithms improving synchrony,        particularly in conditions such as obstructive lung disease        where inspiratory flow is not an accurate indicator of the start        of inspiratory effort.

(viii) Using venous pulsation as an input to ResMed's AUTOSET CPAPalgorithms for patients with OSA and heart failure. Increases in venouspulsation can be used to limit the CPAP pressure applied to saferlimits.

(ix) Using vascular compliance as an input to ResMed's CPAP algorithms.Changes in vascular compliance can be indicative of patient arousals.This can be used to augment the data currently used for automaticallyprescribing CPAP levels.

(x) Comparison of the respiratory effort estimate with the respiratorydevice's own estimate of breath phase (parameter used in ResMed'sAutoCS2 and AutoVPAP) may allow a more robust breath-tracking schemewithin the respiratory device; for example, it may improve leakrejection or leak estimation.

(xi) Sleep state—inference of sleep structure, sleep onset and sleeptermination.

Analysis of the plethysmographic waveshape, possibly in combination withother monitored variables, may be used to optimize CPAP or VPAPtherapies to reduce arterial stiffness, independently associated withpoor cardiovascular prognosis. For example:

(i) Calculation and trending of pulse-transit time (method outlined in[0057] above). Accurate PTT estimation may offer additional informationto that of the plethysmograph alone, contributing to the estimation ofarterial tone/SNS activity and/or respiratory effort, and allowingclosed-loop therapies aiming to optimise arterial compliance.

(ii) The morphology of the plethysmographic waveform may offerinformation directly associated with vascular compliance, for example,the position of the so-called ‘diochrotic notch’ relative to the initialsystolic peak, allowing closed-loop therapies aiming to optimisearterial compliance.

With reference to FIG. 3-7 it is noted that:

THERAPY ALGORITHM adjustments may include:

-   -   Level of PEEP/CPAP    -   Level of Pressure support

Concerning the two feedback signals F/B1 and F/B2 it is noted that:

F/B1 (Airflow-inferred patient parameter) may include any or all of thefollowing:

-   -   Minute ventilation estimate    -   Inspiratory airflow limitation (e.g., UA flattening index)    -   Expiratory airflow limitation (e.g., expiratory flow waveform        morphology)    -   Tidal volume    -   Leak    -   Cardiac timing (time of systolic ejection, extracted from        cardiogenic flow)    -   Respiratory phase

F/B 2 (PPG-inferred patient parameter) may include any or all of thefollowing:

-   -   Relative indication of respiratory effort (e.g., high effort        leads to increased respiratory baseline variation of PPG, pulsus        paradoxus)    -   Absolute indication of respiratory rate    -   Patterns of respiratory effort and rate indicative of        respiratory control anomalies or apnea type        (crescendo/decrescendo in breathing effort, statistical        derivations from respiratory patterns)    -   Indication of respiratory rate (e.g., variation of PPG amplitude        and timing parameters)    -   Relative indication of worsening cardiac function (e.g., cardiac        decompensation results in increased respiratory baseline        variation of PPG, pulsus paradoxus)    -   Relative indication of venous congestion (e.g., degree of venous        pulsation in PPG-morphological analysis)    -   Relative variation in sympathetic nervous system activity or        arterial compliance (e.g., variation of PPG pulse amplitude        over >20-30 second timescale, or shift in location of dicrotic        notch)    -   Standard pulse oximetry (SpO₂)    -   Arrival time of systolic pulse at periphery (e.g., systolic rise        in PPG).    -   Pulse rate

CLINICAL TARGETS may include:

-   -   Minimum ventilation (e.g., Respiratory Insufficiency, Obesity        Hypoventilation patients)    -   Nominal ventilation (e.g., Cheyne-Stokes Respiration patients    -   Optimal synchrony    -   Sleep quality (all patients)    -   Long-term cardiac function (e.g., CHF/CSR/hypertensive        patients).    -   Anticipation/prediction of cardiac decompensation (e.g., CHF        patients)    -   Optimal arterial compliance    -   Minimum CPAP/EEP/PEEP    -   Maximum CPAP/EEP/PEEP    -   Minimum Pressure Support    -   Maximum Pressure    -   Maximum Average Pressure

FIG. 3A is a schematic diagram for a monitoring system according to anembodiment of the present invention. Concerning the feedback signalsF/B1 and F/B2, and the “Combined Processing” box, it is noted that:

F/B 1 (Airflow-inferred patient parameter) may include any or all of thefollowing:

-   -   Inspiratory airflow limitation (e.g., UA flattening index)    -   Expiratory airflow limitation (e.g., expiratory flow waveform        morphology)    -   Cardiac timing (time of systolic ejection, extracted from        cardiogenic flow)    -   Respiratory phase    -   Time course of breath amplitude and derived statistics

F/B 2 (PPG-inferred patient parameter) may include any or all of thefollowing:

-   -   Relative indication of respiratory effort (e.g., high effort        leads to increased respiratory baseline variation of PPG, pulsus        paradoxus), trended over durations relevant to the application        (any amount of time ranging from, for example, a number of        breaths to a number of months).    -   Absolute indication of respiratory rate.    -   Relative indication of worsening cardiac function (e.g., cardiac        decompensation results in increased respiratory baseline        variation of PPG, pulsus paradoxus)    -   Relative indication of venous congestion (e.g., degree of venous        pulsation in PPG-morphological analysis)    -   Relative variation in sympathetic nervous system activity or        arterial compliance (e.g., variation of PPG pulse amplitude        over >20-30 second timescale, or shift in location of dicrotic        notch), as seen during arousal from sleep.    -   Standard pulse oximetry (SpO₂)    -   Arrival time of systolic pulse at periphery (e.g., systolic rise        in PPG).    -   Pulse rate

COMBINED PROCESSING may include:

-   -   F/B 2 alone with no combined processing (e.g., a single        parameter derived from the oximeter signal).    -   Delay between respiration changes (F/B 1) and blood gas        adjustments (F/B 2), eg to infer circulatory delay.    -   Pulse transit time (PTT) indicated by delay between cardiogenic        flow pulses (F/B 1) and arrival of the pulse at the periphery        (F/B 2).    -   Multiple parameters within F/B 2 alone (e.g., respiratory effort        and oxygen saturation).

CLINICAL MONITORING TARGETS may include:

-   -   Assessment of SDB    -   Assessment of sleep quality (all patients)    -   Assessment of cardiac function (e.g., CHF/CSR/hypertensive        patients) as an adjunct to patient management.    -   Early warning of exacerbation of respiratory compromise, as        common in chronic obstructive pulmonary disease or asthma.

FIGS. 4-7 show a number of algorithms performing various embodiments ofthe invention. Embodiments of the invention may take the form of amethod and/or apparatus to monitor, in a non-invasive manner, one ormore parameters, e.g., pulse oximetry and/or air flow, relating, e.g.,to a patient's breathing and/or heart activity.

The monitored parameter or parameters may be used for diagnosticpurposes, e.g., to log data, to produce a report or an alarm orotherwise signal a physician. In addition, or in the alternative, thevalues of the monitored parameter(s) may be used to control, e.g., stop,start or vary, the delivery of pressurized gas (e.g., timing, flowpressure) from a blower, ventilator or the like, to the patient.

FIG. 4 shows an open/closed airway apnea algorithm. An airflow signal isanalyzed and a determination is made as to whether it is within normalbounds. If it is then CPAP/EPAP therapy is maintained at its currentlevel. If the airflow signal is not normal, for example low indicativeof an apnea, then the effort signal is analyzed. If the effort is highthen an obstructive apnea may be indicated and the appropriate therapyis to increase the treatment pressure.

FIG. 5 shows an algorithm for patients suffering general respiratoryinsufficiency. The algorithm defines when pressure support, or EndExpiratory Pressure (EEP) should be varied.

FIG. 6 shows an algorithm which may be part of a monitoring system forevaluating cardiac performance. A cardiac patient may be receiving CPAPtherapy and have an additional monitoring device with the algorithm ofFIG. 6. Alternatively the CPAP device may incorporate the pulseoximeter. The two signals F/B/1 and F/B/2 are analyzed. Where the valuesare indicative of elevated levels of SNS activity, or decompensation(poor cardiac performance) an alert is generated. The alert may be inthe form of an audible alarm, or part of a messaging system whichreports to a physician.

FIG. 7 depicts an algorithm for cardiac patients on CPAP therapy. Thealgorithm is similar to that in FIG. 4. However, it has the additionalstep that venous congestion is monitored through the pulse oximeter. Ifvenous congestion is worsening, then CPAP pressure will not beincreased, but restored to a previous level.

FIG. 8 depicts a procedure for initializing NPPV therapy rate settings,based on respiratory rate information. Preferably, this is performedafter attaching oximeter probe (F/B2), but can be attached prior tocommencing ventilation.

FIG. 9 depicts a procedure for initializing NPPV therapy triggerthreshold settings, based on positively identifying cardiogenic flowamplitude. Preferably, this is performed once ventilation is initiated,e.g., so a respiratory flow signal is available.

The combination of traditional oximetry data (saturation, heart rate,pulse timing information) and respiratory timing and effort information(inferred from additional processing of a PPG and/or from the additionof a nasal or oronasal cannulae data) may permit new diagnosticpossibilities. For example:

-   -   Circulatory delay (delay between breathing changes and        saturation changes), an indicator of heart-failure severity or        cardiac decompensation.    -   ‘True’ Pulse Transit Time (PTT), via the delay between        cardiogenic flow pulses seen by the nasal pressure transducer at        end-expiration (seen at the nares) and the arrival of pulse at        the periphery (from the oximeter plethysmogram), as described        above.    -   By extracting respiratory effort information from the raw PPG        (pulsus paradoxus) a simple diagnostic system may offer all the        key information required for SDB screening except sleep staging:        breathing pattern, oxygen saturation, arousal (PTT) or increased        systemic vascular resistance, and high effort periods (apnea        discrimination and respiratory-effort related arousal (RERA)        classification). This system may optionally include a nasal        pressure transducer, depending on the relative importance of the        derived signals: nasal airflow combined with respiratory effort        permits straight-forward discrimination between central and        obstructive apneas, but conversely with suitable signal        processing, the same information can be gleaned by combining        information from the PPG. For example, if increased relative        breathing effort precedes oxygen desaturation, an obstructive        apnea is discriminated from a central or mixed apnea, in which        the desaturation is not accompanied or preceded by increased        effort. Similarly, classifier or pattern recognition techniques        may be applied to the time course of breathing effort to        distinguish obstructive from central apnea.

Other specific examples of where aspects of the invention may be usedinclude:

(a) Using respiratory-related cardiac rhythm variations (e.g.,“respiratory sinus arrhythmia”) to track and predict breath-phase, andto use the prediction for ventilator triggering. Such variations mayconveniently be detected in the PPG, but may also be detected by othercardiac monitoring devices such as ECG electrodes. Typically therespiratory variation imposed on cardiac timing occurs too late to beused as a ventilator trigger: ventilators ideally offer respiratorysupport coincident with early inspiration, preferably within 100 msec ofthe patient's initial inspiratory effort. Ventilators typically monitorinspiratory flow or airway pressure variations as a trigger. In severeobstructive respiratory disorders (e.g., COPD) the respiratory flow orpressure information is a poor indicator of inspiratory timing. In suchdisorders, an alternative ‘window’ into respiratory activity may offersuperior results. Respiration, particularly labored respiration, isknown to affect cardiac timing and cardiac output. By monitoring cardiacperformance over previous breath cycles, and deriving a respiratoryphase signal from cardiac information, the timing of the nextinspiratory effort can be predicted, provided the latency of extractingthe respiratory signal is not excessive (e.g., more than 1 breathdelayed). The central-to-peripheral propagation time for the pulse istypically around 200 msec (the “pulse transit time”), and at best thecardiac cycle would offer a low sample-rate estimate of breath phase(about 4-6 beats per breath). So it is unlikely that a prediction ofstart of inspiration will not offer precise inspiratory timing. However,such a method still offers significant utility in disease states such asCOPD, where ventilator synchronisation via respiratory flow is typicallyvery delayed, and where breath timing may be more entrained than innormal breathing (and therefore predictability being potentiallygreater).

(b) Using heart-period analysis to infer sleep onset within asleep-disorder screener device.

(c) A number of measures also can aid in clinical management of home,chronic ventilation, and/or CPAP patients. For example, tracking sleepstructure (e.g., start, finish, REM extrapolated from HRV analysis,etc., as described above) within a therapy device can indicate thetherapy's effectiveness. Patient-ventilator asynchrony can be extractedfrom, for example, a PPG spontaneous effort signal. A patient'sautonomic improvement in response to therapy (e.g., CPAP) can bemeasured based on HRV analysis and also can be monitored. An index hasbeen published (Khoo et al., Cardiac Autonomic Control in ObstructiveSleep Apnea—Efects of Long-term CPAP Therapy. Am J Respir Crit Care MedVol 164. pp 807-812, 2001) that shows a dosage response between CPAP andautonomic nervous activity (e.g., sympathetic and parasympathetic). Theindex may highlight the benefits of CPAP therapy in minimizing the riskof further adverse vascular events. The index is based on heart-ratevariability (originally as acquired by ECG), corrected for the effect ofrespiratory effort on heart-rate. The PPG signal possesses informationon both, so it may by suited to provide this index for long-termtrending.

(d) Clinical management and prediction of respiratory exacerbations maybe possible, in part, for example, because a pulse oximeter iscomfortable and sufficiently easy to use for it to be a routine,long-term home nocturnal monitory. As such, for any chronic respiratorydisorder associated with exacerbations that have progressive onset,early detection may be possible if there was a device that could inferrelative breathing effort, breathing frequency, breathing phase(including inspiratory and/or expiratory timing). The PPG signal mayprovide this, and coupled with the inherent arterial oxygenation, mayoffer a coarse representation of respiratory efficiency (e.g., outputvs. input). Applicable conditions may include, for example, asthma,COPD, and the like.

(e) Assessment of endothelial dysfunction: conditions such asrespiratory-related arousal during sleep can be considered aninvoluntary intervention to provoke a sympathetic response. The degreeof vasoconstrictive response seen in the PPG acquired from a fingerpulse oximeter, trended over a given period (e.g., days, weeks, and/ormonths), may indicate change in endothelial function, which may be amarker of improving or worsening patient status (e.g., the onset ofpre-eclampsia, etc.). The arousal may be entirely spontaneous, or if thepatient is on CPAP therapy, can be periodically invoked via a CPAPpressure step-down.

(f) Detection/diagnosis of periodic breathing, (e.g., in cardiac failurepatients), evident as periodic variation in the relative respiratoryeffort signal.

(g) In a ventilator system equipped with customized PPG monitoring,detecting dramatic drop in cardiac output (inferred from PPG amplitudereductions), and asserting an alarm. A drop in cardiac output may be aconsequence of many clinically relevant circumstances, e.g., applyingexcessive positive pressure in a patient with hypovolemia (Yamakage, CanJ Anesth 2005 52(2): 207), excessive dynamic hyperinflation/air trapping(Perel, B J A 76(1):168-169) (Conacher, Lancet 1995 346:448).

Advantages for the patient include, for example, more comfort and easeof use. Aspects of the invention provide optimal therapy without beingfestooned with sensors, e.g., a finger or ear probe is sufficient.Advantages for the physician include, for example, ease to administer.Aspects of the invention provide simple application, automatedtherapies, and long term patient management feedback. Other advantagesinclude less expensive and improved therapy.

Although the invention has been described with reference to particularembodiments, it is to be understood that these embodiments are merelyillustrative of the application of the principles of the invention.Numerous modifications may be made therein and other arrangements may bedevised without departing from the spirit and scope of the invention.For example, those skilled in the art recognize that there are otherindications of upper airway instability, resistance or obstruction whichare not necessarily accompanied by or associated with flow flattening.

Also, the various embodiments described above may be implemented inconjunction with other embodiments, e.g., aspects of one embodiment maybe combined with aspects of another embodiment to realize yet otherembodiments. In addition, while the invention has particular applicationto patients who suffer from OSA, it is to be appreciated that patientswho suffer from other illnesses (e.g., congestive heart failure,diabetes, morbid obesity, stroke, bariatric surgery, etc.) can derivebenefit from the above teachings. Moreover, the above teachings haveapplicability with patients and non-patients alike in non-medicalapplications.

What is claimed is:
 1. An air delivery system, comprising: acontrollable flow generator operable to generate a supply of pressurizedbreathable gas to be provided to a patient for treatment; a pulseoximeter; and a controller configured to determine sleep structureinformation for the patient and track the sleep structure information toindicate a therapy's effectiveness based at least in part on output fromthe pulse oximeter.
 2. The air delivery system of claim 1, wherein thesleep structure information includes information about the patient'sstart, finish, and/or REM sleep states.
 3. The air delivery system ofclaim 2, wherein information about the patient's sleep state isextrapolated via heart period analysis.
 4. The air delivery system ofclaim 1, wherein the controllable flow generator is adapted to provideContinuous Positive Airway Pressure (CPAP).
 5. The air delivery systemof claim 1, wherein the sleep structure information is tracked byperforming statistical or fractal analysis of pulse interval data. 6.The air delivery system of claim 1, wherein the sleep structureinformation is tracked by analyzing heart-rate variability indices toindicate sleep onset.
 7. A method for diagnosing sleep disorderedbreathing, said method comprising: deriving a pulse oximeter signal froma patient; processing the pulse oximeter signal to generate a patienteffort signal indicative of respiratory rate; and tracking sleepstructure information for the patient to indicate a therapy'seffectiveness.
 8. The method of claim 7, wherein the sleep structureinformation includes information about the patient's start, finish,and/or REM sleep states
 9. The method of claim 8, wherein informationabout the patient's sleep state is extrapolated via heart periodanalysis.
 10. The method of claim 7, wherein the sleep structureinformation is tracked by performing statistical or fractal analysis ofpulse interval data.
 11. The method of claim 7, wherein the sleepstructure information is tracked by analyzing heart-rate variabilityindices to indicate sleep onset.
 12. The method of claim 7, wherein thepatient effort signal is based upon an arterial blood pressure waveformvariation in peak-to-peak amplitude.
 13. A respiratory effort monitoringapparatus, comprising: a pulse oximeter configured to derive a pulseoximeter signal; and a signal processor configured to receive the pulseoximeter signal and generate a patient effort signal indicative ofrespiratory rate; wherein the signal processor is configured to tracksleep structure information for the patient to indicate a therapy'seffectiveness.
 14. The respiratory effort monitoring apparatus of claim13, wherein the sleep structure information includes information aboutthe patient's start, finish, and/or REM sleep states.
 15. Therespiratory effort monitoring apparatus of claim 14, wherein informationabout the patient's sleep state is extrapolated via heart periodanalysis.
 16. The respiratory effort monitoring apparatus of claim 13,wherein the signal processor is configured to trend sleep structureinformation for the patient to indicate a therapy's effectiveness over aperiod of time.
 17. The respiratory effort monitoring apparatus of claim13, wherein the sleep structure information is tracked by performingstatistical or fractal analysis of pulse interval data.
 18. Therespiratory effort monitoring apparatus of claim 13, wherein the sleepstructure information is tracked by analyzing heart-rate variabilityindices to indicate sleep onset.
 19. The respiratory effort monitoringapparatus of claim 13, wherein the patient effort signal is based uponan arterial blood pressure waveform variation in peak-to-peak amplitude.20. The respiratory effort monitoring apparatus of claim 13, furthercomprising a filter adapted to filter heart rate out of the pulseoximeter signal.